I think you're on to something with the "can only hold one key at a time" concept. I'd recommend trying something less procedurally generated and do some custom puzzles with the system at first to get a feel for what makes the game design tick. Maybe it's like you start with two keys in the room and you only need one, then you move on to having to use both keys to get to the end, then three keys, and so forth. Meanwhile the adversaries become more difficult.
Also, nice touch with the floating hospital bed frames.